An active matrix liquid crystal display which is also described as a TFT-LCD (thin film transistor liquid crystal display) due to the controllable switches commonly in the form of thin film transistors, is described in U.S. Pat. application Ser. No. 4,635,127.
The conventional active matrix liquid crystal display has pixels arranged in rows and columns of a matrix which are formed by pixel electrodes mounted on a common carrier, a reference electrode which is common to all pixel electrodes and lies opposite them, and a liquid crystal layer located between the pixel electrodes and the reference electrodes. The pixel electrodes are connected column-by column to column electrodes via controllable switches assigned to them individually, the switches being connected to a column control unit generating various gray-scale signals for the different columns. On the control side, the controllable switches are connected row-by-row to row electrodes which are connected to a row control unit generating turn-on signals in succession for the switches in the individual rows. The terms rows and columns are interchangeable here as well as below.
To display images with various gray-scale values, gray-scale signals are applied simultaneously to all column electrodes, the gray-scale signals representing the gray-scale values of one row each; via a turn-on signal at one of the row electrodes, the gray-scale signals are switched through to the pixel electrodes of the affected row. In this manner, all rows with pixels are activated in rapid succession. The optical transparence of the liquid crystal cells, i.e., of the liquid crystal layer, is set as a function of the voltage between the particular pixel electrode and the reference electrode so that the desired image is displayed with background lighting of the active matrix liquid crystal display. To avoid gray-scale value corruptions in image rendering, the liquid crystal cells are operated in a voltage range in which the otherwise typically non-linear dependence between the transparence of the liquid crystal cells and the voltage applied to them is approximately linear.
For the display of color images, red, green and blue color filter strips are arranged in front of or behind the liquid crystals alternately column-by-column, the three adjacent liquid crystal cells lying behind or in front in a row being combined into one color pixel made up of three sub-pixels as regards their activation. In rendering color images, non-linearities between the transparence of the liquid crystal cells and the voltage applied to them can have a particularly interfering influence.
The transparence of each individual liquid crystal cell occurring as a function of the applied voltage depends on the viewing angle due to the voltage-dependent optical twisting of the liquid crystal, so that with a specific voltage applied to the liquid crystal cell, the brightness of the displayed pixel varies as a function of the viewing angle of the observer.
The use of this effect is conventional in liquid crystal displays which are designed only for bright/dark or black/white display, but not for displaying different brightness and gray-scale values. An example of this is the setting of optimum contrast ratios for a specific viewing angle. An additional example described in U.S. Pat. No. 5,526,065 is the use of such a liquid crystal display as an optical filter in front of a conventional screen in a vehicle to make the image displayed invisible in the driver""s viewing angle during travel but visible to the front-seat passenger.
An object of the present invention is to improve and expand the possibilities of image rendering using an active matrix liquid crystal display without increasing the number of rows and columns to be activated.
In the active matrix liquid crystal display of the present invention, each pixel is thus divided into at least two sub-pixels. To be able to activate all sub-pixels of the particular pixel independently via only one row electrode and one column electrode, a control signal is given to the row electrode which is converted at the point of the pixel via the pixel control unit located there into locally and chronologically different turn-on signals for the controllable switches assigned to the sub-pixels. During the various turn-on times in which the controllable switches are turned on, various gray-scale signal levels are applied at the column electrode so that the liquid crystal sub-cells formed by the areas of the liquid crystal layer between the sub-pixel electrodes and the reference electrode are charged to different voltages which are retained until the next activation. The activation of the sub-pixels thus takes place more or less at the same time with that of the pixels and accordingly without any appreciable delay. The optical transparence of each of the liquid crystal sub-cells is thus a function of the voltage applied to it.
The active matrix liquid crystal display of the present invention thus makes it possible to display considerably more image information than is the case with previous active matrix liquid crystal displays with the same number of row and column electrodes. Since in the active matrix liquid crystal display of the present invention all sub-pixels of each pixel are activated only via one single row electrode and one single column electrode, in the active matrix liquid crystal display of the present invention, the number of the required row and column electrodes is conversely less than in conventional active matrix liquid crystal displays with the identical image resolution by a factor corresponding to the number of sub-pixels per pixel so that the surface area of the active matrix liquid crystal display of the present invention which is opaquely covered by the row and column electrodes and consequently not available for the image display is less and, in addition, the amount of circuitry required for the row and column control unit is less.
In the context of the present invention, the control signals on the row electrodes can be converted into locally and chronologically different turn-on signals in various ways.
Thus, each control signal may be made up of chronologically successive signal pulses which are converted into the locally and chronologically different turn-on signals in the pixel control unit by means which are suitable and conventional such as shift registers, counters or serial-parallel converters. To prevent the sequence of the turn-on signals generated for the sub-pixel electrodes from being changed, for example, by interference pulses on the row electrode, each control signal preferably also contains a synchronizing pulse, the form, intensity, duration or polarity of which is different from the other signal pulses and which is used to synchronize the signal conversion for each new activation of the sub-pixels.
Alternatively for the signal conversion, each control signal is made up of chronologically successive sub-signals having different signal levels and each pixel control unit has suitable means which are conventional, such as threshold value or window comparators, for recognizing and converting the various signal levels into locally and chronologically different turn-on signals. In the simplest case, such means are made up of electrical components having a threshold value characteristic, for example, diodes.
If each pixel is always made up of only two sub-pixels, the control signal maybe made up of two signal pulses or sub-signals which differ from each other in form, intensity, duration or polarity.
As mentioned above, the active matrix liquid crystal display of the present invention makes it possible to display considerably more image information than is the case with the previous active matrix liquid crystal displays with the same number of row and column electrodes.
Thus, various color filters can be connected upstream or downstream of the individual pixels for the display of color images. The color of each pixel is set via the different gray-scale levels of the gray-scale signal with which the do individual sub-pixels are activated, the gray-scale levels for each of the sub-pixels being equal for pure bright/dark or black/white displays.
An additional advantage of the active matrix liquid crystal display of the present invention lies in the possibility to adjust the viewing angle range within which the displayed image is visible while adhering to specific contrast conditions. As mentioned above, the optical transparence of each liquid crystal cell or liquid crystal sub-cell to be set as a function of voltage is dependent on the viewing angle. In the active matrix liquid crystal display of the present invention, the varying activation of the sub-pixels makes it possible to adjust the size and position of the viewing angle range within which the individual pixel appears with a specific brightness. For this purpose, the active matrix liquid crystal display of the present invention has an adjustment device with which the chronologically successive gray-scale levels for the individual sub-pixels of each pixel can be changed while at least approximately preserving the average gray-scale level of all such sub-pixels. In this manner, the viewing angle-dependent brightness levels of the individual sub-pixels are changed while the total brightness of the pixels formed from them is as a whole unchanged.
Since, as already mentioned above, the dependence between the transparence of the individual liquid crystal cells and liquid crystal sub-cells and the voltage applied to them is typically non-linear, the liquid crystal sub-cells are either operated in a voltage range in which this dependence is approximately linear or the gray-scale signals supplied to the liquid crystal sub-cells are distorted in a correction device on the basis of information concerning the typical dependence between the transparence of liquid crystal sub-cells and the voltage applied to them and as a function of the gray-scale levels in such a manner that an at least approximately linear relationship arises between the transparence of the liquid crystal sub-cells and the undistorted gray-scale signals. In the later case, the contrast conditions for specific viewing angle ranges can be set via a largely unlimited selection of the voltage range in which the liquid crystal sub-cells are operated without the occurrence of corruptions of the image rendering.
When the active matrix liquid crystal display of the present invention is used in a vehicle, it is thus possible to hide the displayed image in the viewing angle range of the vehicle driver in the same manner as is described in the aforementioned U.S. Pat. No. 5,526,065, while it is visible to the front-seat passenger; in contrast to the conventional methods, image rendering takes place directly via the active matrix liquid crystal display which is by far better suited for use in vehicles than conventional screens due to, among other things, its smaller mounting depth. An additional possibility for use of the active matrix liquid crystal display of the present invention is the display of three-dimensional objects, different views of one and the same object being displayed for different viewing angle ranges which can be set.
To be able to display different images for different viewing angle ranges in an advantageous manner, it is provided in connection with the present invention that the column control unit has means for the sequential arrangement of the gray-scale signals supplied by at least two different image signal sources in the gray-scale signals generated by it so that sub-pixels of each pixel are activated with the gray-scale levels of at least two different images.
With the simultaneous rendering of different images in different viewing angle ranges, it is, for example, possible in a motor vehicle to display traffic information to the driver and to present a video film to the front-seat passenger at the same time. In a like manner, it is possible to show different images (videos) via a single active matrix liquid crystal display to passengers sitting side-by-side in rail coaches or airplanes.
As stated above, the viewing angle range within which the pixels are each visible with a specific brightness can be changed by the different activation of the sub-pixels of the individual pixels; in particular, the displayed image can be hidden in specific viewing angle ranges in this manner or different images can be displayed in different viewing angle ranges. As a supplement or as an alternative to this, it can be provided in connection with the present invention that the liquid crystal layer is divided in the area of each individual pixel into domains assigned to the individual sub-pixels with varying orientations of the liquid crystal. For the simultaneous display of two different images in two different viewing angle ranges, for example, the sub-pixels for which the liquid crystals lie in a first direction are activated with the gray-scale levels supplied by a first image signal source and the sub-pixels for which the liquid crystals lie in a second direction are activated with the gray-scale levels supplied by a second image signal source. Accordingly, for the display of a single image of the viewing angle range within which the image is visible, the change can be made that either the sub-pixels with the liquid crystals lying in the first direction or the sub-pixels with the liquid crystals lying in the second direction or all sub-pixels are activated. A suitable number of sub-pixels or individual activation of the sub-pixels thus makes it possible to display different images in different viewing angle ranges and to change the viewing angle ranges.
The active matrix liquid crystal display offers the advantage that a defect in one individual sub-pixel can be compensated by changing the activation of the remaining sub-pixels so that the overall brightness of the pixel made up of the affected sub-pixels does not change.